Conventional press bending of a glass sheet is typically accomplished by heating the glass sheet to its bending temperature, positioning the glass sheet between a pair of cooperating press bending molds having complimentary shaping surfaces, bringing the shaping surfaces against the glass sheet and subsequently applying pressure to press bend the glass sheet so that it conforms to the shape of the shaping surfaces.
Typically one bending mold is convex and has a full surface, and the other is concave and is in the form of a ring. When the shaping surfaces have a non-abrupt curvature, the resultant bent glass sheet usually has a predictable contour and good visual properties. However, when the shaping surfaces include an abrupt curvature, the contour of the resulting bent glass sheet is not predictable and its visual properties are typically disfavorable. This unpredictable contour results because negatively curving areas are formed in the bent glass sheet. These negatively curving areas are created in the areas adjacent the abrupt curvature when the glass sheet separates from the convex surface, as the concave ring shape is pressed toward the convexly shaped surface during the bending. A fulcrum is created by the crease in the convex shaped surface and the glass is strong enough to resist bending over that crease. On either side of the fulcrum, separation of the glass sheet from the shaping surface is likely as the glass sheet is bent.
U.S. Pat. No. 4,229,200 discloses a glass bending apparatus for use in drop forming glass sheets which includes a downwardly facing vacuum through which a vacuum is applied at a pair of spaced edge chambers and an intermediate center chamber. Such a construction is stated in the patent to allow the central portion of the glass sheet to be released either before or after its edges in order to control the manner in which bending takes place.